The present invention relates generally to the field of time domain reflectometers, and more particularly to a time domain reflectometer with a digitally generated variable width pulse output.
Time domain reflectometry is commonly used within the telephone and cable television industries. A Time Domain Reflectometer (xe2x80x9cTDRxe2x80x9d) sends a pulse down a transmission line and then monitors the transmission line for any reflections of that pulse. Reflections are caused by changes in impedance along the transmission line. A change in impedance may indicate the presence of a fault. As a pulse transmitted by a TDR reaches the impedance mismatch, a portion of the transmitted pulse is reflected back to the TDR. Because the transmitted and reflected pulses travels along the transmission line at a known speed of propagation, the exact location of the impedance mismatch may be determined by measuring the time at which the transmitted pulse is transmitted and the time at which the reflected pulse is received by the TDR.
The magnitude of the reflected pulse is proportional to the magnitude of the impedance mismatch. The sign or polarity of the reflected pulse is determined by the direction of the change in impedance. For example, if the transmitted pulse is positive and the impedance at the fault increases, then the reflected pulse will be positive. A break in the line will result in strong positive reflected pulse. If the transmitted pulse is positive and the impedance at the fault decreases, then the reflected pulse will be negative. For example, a short in the line will produce a negative reflected pulse. Thus, the nature of the fault may be determined or inferred from analysis of the reflected waveforms.
The energy of the transmitted pulse is dependent on the width of the pulse. The larger the pulse width, the more energy is transmitted and therefore the further the signal will travel down the line. Accordingly, many currently available TDRs have a limited number selectable pulse width settings. Each pulse setting produces pulses of substantially identical width. Thus, each pulse of a selected width has a substantially identical frequency spectrum, which can result in electromagnetic interference with digital services on the line.
Conventional TDR pulse generation technology uses low speed logic to generate pulses or analog RLC circuits to generate half-sine wave transmitted pulses. The rise time of conventionally generated pulses is relatively slow, thereby making it very difficult to interpret reflections from some types of faults such as water in the cable, bridge taps, untwisted cable, etc. The slow rise time problem is particularly acute when using long half-sine wave transmitted pulses.
The present invention provides a digital variable width pulse generator that finds particular application in a time domain reflectometer. The pulse generator of the present invention includes circuitry for starting a pulse in response to receipt of a pulse enable signal. The pulse enable signal is synchronous with a first time base. For example, the time base for the pulse enable signal may be 5.529 MHz. The pulse generator includes circuitry for ending said pulse a predetermined, user selectable, number of clock cycles after the pulse enable signal. The clock cycles are have a second time base that is asynchronous with the first time base. For example, the time base of the clock signal may be 80 MHz. Since the end pulse signal is not on the same time base as the pulse enable signal, there is up to one asynchronous clock cycle period random variation in width of pulses having the same nominal width. For example, in the 80 MHz example, a nominal 100 nanosecond pulse may range continuously from 100 nanoseconds to 112.5 nanoseconds in width. The random variation in pulse width produces a spread spectrum effect.